The invention relates generally to gas turbine engines and, more particularly, to an integral environmental shield and impingement manifold for supplying cooling air to the exterior surface of a casing of a gas turbine engine.
It is understood in the gas turbine art that engine efficiency is improved by minimizing the leakage of hot gases past the turbine. Leakage air does not contribute to the power extracted by the turbine and consequently represents a loss of overall efficiency. Therefore, much effort has been given to limiting clearance between rotor and stator components.
Typically, the prior art has supplied cooling air to gas turbine components to control thermal growth of the turbine casing to minimize the operating clearances. For example, cooling air is supplied to circular spray bars which impinge cooling air upon stator components surrounding a row of turbine blades. Prior art U.S. Pat. No. 4,214,851, issued Jul. 29, 1980, to Tuley, et al., and assigned to the assignee of the present invention, discloses a cooling air manifold of an annular shape with radial holes to supply cooling air to stator components. To further control cooling, the air supply has been controlled as described in, for example, U.S. Pat. No. 4,230,436, issued to Davison and assigned to the assignee of the present invention. In Davison, two sources of air are mixed according to the demand to provide a cooling flow in response to measured engine operating parameters. Another requirement for clearance control for a gas turbine is to shield the turbine stator from air currents of unknown temperature and velocity within the engine nacelle from impinging directly on the stator casing, because such air could create a "cold spot" which would cause distortion of the casing, thereby adversely affecting clearance control.
An improved turbine casing cooling manifold is disclosed in U.S. Pat. No. 5,100,291, issued Mar. 31, 1992, to Jeffrey Glover, and assigned to the assignee of the present invention. That patent discloses apparatus to spray cooling air over flanges of a gas turbine stator structure through arcuate segment tubes disposed around a gas turbine stator to apply uniform cooling to the stator components. A cooling pattern is selected to provide optimum cooling, and a baffled construction is used to reduce distortion due to external influence on the stator. The baffled construction also provides controlled passage of spent impingement air out of the turbine. A separate environmental shield was provided to isolate the stator from the external environment. This construction enhanced uniformity of cooling around the circumference of the turbine casing.